The present invention relates generally to electronic memory and, more particularly, to a ferroelectric memory device and a method of reading such a device.
Ferroelectric memory is a type of nonvolatile memory that utilizes the ferroelectric behavior of certain materials to retain data in a memory device in the form of positive and negative polarization, even in the absence of electric power. A ferroelectric material contains domains of similarly oriented electric dipoles that retain their orientation unless disturbed by some externally imposed electric force. The polarization of the material characterizes the extent to which these domains are aligned. The polarization can be reversed by the application of an electric field of sufficient strength and polarity.
FIG. 1 illustrates an exemplary known ferroelectric cell 10 in a ferroelectric memory array. A ferroelectric material 16 having a polarization P is sandwiched between a conductive word line 20 and a conductive bit line 22. An electric field may be applied to the ferroelectric cell by applying an electric potential (voltage) between the word line and the bit line so as to effect changes in the polarization of the ferroelectric material.
FIG. 2 shows a simplified hysteresis curve 24 that illustrates idealistically the polarization versus voltage properties of the exemplary ferroelectric cell of FIG. 1. When a positive voltage of sufficiently large magnitude (shown here, for example, as Vs) is applied to the cell, all of the domains in the cell are forced to align, to the extent possible, in the positive direction, and the polarization P reaches the saturation polarization Psat at point 25 on the curve. A further increase in the voltage produces no further increase in the polarization because all of the domains are already aligned as far as possible in the direction of the electric field produced by the voltage between the word line and bit line.
If the voltage is then reduced to zero (following path 32 to arrive at point 25), some of the domains switch their orientation (also referred to as rotating, flipping or reversing), but most of the domains retain their orientation. Thus, the ferroelectric material retains a remnant polarization Pr.
If a negative voltage of sufficiently large magnitude (shown here, for example, as xe2x80x94Vs) is then applied to the word line 20 relative to bit line 22 (following path 34 to point 27), all of the domains are forced to switch their orientation, and the polarization reaches the negative saturation level xe2x88x92Psat. Removing this negative voltage (following path 36 to point 23) allows some of the domains to switch, and the cell polarization reaches the negative remnant polarization xe2x88x92Pr, which it retains until it is disturbed again. If the positive voltage Vs is again applied to the cell (following path 30 to point 25), the domains once again switch their orientation, and the cell takes on the positive saturation polarization Psat.
For purposes of data storage, the ferroelectric cell 10 is considered to be in the logic xe2x80x9c0xe2x80x9d (zero) state when the polarization P is positive (preferably at Pr), and the logic xe2x80x9c1xe2x80x9d (one) state when the polarization is negative (preferably at xe2x88x92Pr).
A certain amount of charge is required to switch the polarity of a domain, so the further the polarization moves along the P axis in FIG. 2, the more domains that are switched and the more charge is required. Thus, the transition from the logic 1 state at point 23 to the logic 0 state at point 25 is accompanied by a substantial release of charge, whereas the transition from point 21 to 25 (no change of state) is accompanied by very little charge release.
This difference in charge release provides the fundamental principle for a xe2x80x9cdestructivexe2x80x9d read of a ferroelectric cell. That is, a positive voltage sufficient to switch the polarization is applied to the cell while the charge released from the cell is observed. A large charge release indicates that the cell was a logic one, whereas little or no charge release indicates that the cell was a logic zero. The cell ends up in the zero state, regardless of its state before the read operation. Thus, a cell that was in the one state must then be rewritten as a one if further data retention is required.
Ferroelectric materials also exhibit resilience, wherein a ferroelectric cell can return to its remnant polarization despite a small disturbance. For example, assuming a one state storage condition for a ferroelectric cell, as represented by remnant polarization position 23 of hysteresis curve 24, a small voltage disturbance of Vs/3 provides a small polarization shift 40 along path 38. However, once the voltage is removed, domains of the ferroelectric cell realign their orientations to that of the cell""s overall orientation, as illustrated by return path 39 of hysteresis curve 24.